Generally, a Hybrid Electric Vehicle (HEV) has an idling stop system to improve gas mileage. Such an idling stop system stops an engine when the engine is in an idling stop condition, and automatically operates the engine when the engine is restarted.
In the case of vehicles having brakes operated by compressed air (the majority of hybrid commercial vehicles, hereinafter referred to simply as commercial vehicles), among HEVs, the idling stop on a level surface, rather than a sloped one, is controlled through a method similar to that of general hybrid electric vehicles, but the situation on a slope is different. That is, in the above-described vehicle, air is compressed by a compressor and the compressed air is stored in an air tank, and then the compressed air is used when the brake is operated. The compressor is coupled to the driving axle of an engine and then operated, thus the compressor cannot generate a sufficient amount of compressed air when the engine is idling-stopped.
As a result, when the engine is idling-stopped in the situation in which a sufficient amount of compressed air is not ready, the vehicle moves or is pushed forwards or backwards while being operated on a slope, thus causing a serious problem in safety.